Structural health management is a critical function for mission readiness and cost effectiveness of aircraft operations. Traditionally structural health management has been implemented through scheduled maintenance programs with appropriate nondestructive inspection for defect detection in critical areas. While this approach has been effective in providing low risk of structural failure in flight, it reduces operational availability and incurs significant levels of cost performing inspections that most frequently find no defects.
To reduce the need for removing aircraft from service for periodic scheduled inspections, concepts related to condition based maintenance principles are being developed. Along with advanced prognosis techniques to evaluate and predict Structural Integrity, a significant reduction or elimination of the need for scheduled maintenance will improve fleet life cycle costs and increase the operational availability of aircraft. This approach is made possible by structural health monitoring.
Material state awareness for aircraft structure has traditionally been determined from the detection and measurement of cracks or other damage that represent significant structure feature changes. Normally these feature changes must be of sufficient size that standard nondestructive inspection methods such as ultrasound, radiography and eddy current can measure them. X-ray diffraction can be used to characterize residual stress and this is a measurement in tune with material property or state measurement. Some mode of ultrasound also can be used to return material properties such as Young's modulus.
Another way in which the material state may be determined is by measuring the work function, from which may be derived stress, strain, fatigue, corrosion, adsorption or desorption of molecular species, moisture, etc. Work function is a term applied to the amount of energy required to transfer electrons from the interior of one substance across an interface boundary into an adjacent area of space, commonly expressed in the units of electron-volts. An apparatus for determining work function that is known in the art is the Kelvin probe, the method of which is also called the dynamic capacitor method. Referring to FIG. 1, a Kelvin probe works by placing a conducting plate adjacent to a conductive surface to be tested, arranged as a parallel plate capacitor with a small spacing, on the order of hundreds of microns or even smaller. The conductive plate typically is made of a stable metal such as gold and thus serves as a reference. When an external electrical circuit is connected to the capacitor, a flow of electrons will pass through this circuit, and the electrochemical potentials of the conducting plate and metal surface are equalized thereby. The work function may be measured as an electro-static potential difference generated between the two surfaces equal to the difference between the work function of the respective surfaces. The electro-static potential between the two plates is given by Vcpd=1/e(φ2−φ1), where φ1 and φ2 are the work functions of the metal surface and the conductive plate. Note that the work function is independent of the spacing between the two surfaces and the capacitance is dependent upon said spacing. Thus, a periodic vibration changing the distance between the plates at a frequency ω results in a current i(t)=VcpdωΔC cos ωt, where ΔC is the change in capacitance. For the measurement of Vcpd, an additional backing voltage is applied between the plates until the current i(t) goes to zero. This backing voltage is proportional to the work function difference of the two plates.
The relative work function measurement is very sensitive to any changes on the surface of the material and is a useful tool for structural health monitoring. If corrosion products are formed on the surface, or dislocation density changes due to fatigue occur then the work function will be altered. It has been demonstrated that even wear at very low loads that involves the absence of wear debris and/or wear scars, affects the work function. The change in surface potential is believed to be the result of chemical and structural changes in the first few nanometers of the sample. Hence a Kelvin Probe should be a very sensitive sensor for surface changes and health monitoring. See, for example, U.S. Pat. No. 4,649,336; Derik DeVecchio and Bharat Bhushan, “Use of a Nanoscale Kelvin Probe for Detecting Wear Precursors,” Rev. Sci. Instrum. 69, 3618 (1998); Kenichi Takahata and Yogesh B. Gianchandani, “Bulk-Metal-Based MEMS Fabricated by Micro-Electro-Discharge Machining,” Electrical and Computer Engineering, 2007; and I. D. Baikie and P. J. Estrup, “Low Cost PC Based Scanning Kelvin Probe,” Rev. Sci. Inst. 69(11), 3903 (1998).
Structural health monitoring technology currently has several shortcomings relative to the principles of non-destructive inspection. Structural health monitoring has not been proven to have reliable damage feature resolution with acceptable detection rates and at acceptable weight and cost. Furthermore, both structural health monitoring and non-destructive inspection methods are limited in terms of providing direct measurement of material state and often require baseline information to justify heuristic techniques that search for localized changes from baseline states. Material state awareness is not only knowledge of structural features (such as flaw size or presence of a corrosive environment) but also the condition of the material and its ability to perform its designed function. To achieve these goals, sensors, their power requirements and wiring must be such that they can be justified when weight and performance are critical to mission effectiveness.
Prior art devices do not address the problem of determining the material state of an aircraft structure in-service. The existing solutions have difficulties with the size of the equipment for implementation. X-ray diffraction, for example, in field applications is cumbersome and slow. Ultrasound Young's modulus measurement represents only one characteristic of material property measurement. Existing Kelvin probe designs can measure material work functions, but are likewise large and cumbersome.